It is known that most current wireless communication technologies are unable to traverse a thick solid barrier made from materials such as rock, concrete or soil. At frequencies greater than a few kHz, the skin depth of these weakly conductive materials is quite small and electromagnetic waves are attenuated to the point where communication is not possible over even short distances (<1 m). In work environments such as tunnels and mines, workers are therefore not able to readily communicate, for example, with supervisors or other workers located at a different part of the tunnel or located above the surface of the earth.
Such communication systems are not only useful for regular day-to-day work; they are indispensable in times of emergency. For example, if a tunnel collapses with a worker inside, providing such communications could significantly reduce the time to find and rescue the trapped worker. This would provide a significant improvement in the chances of saving the life of the worker.
A number of solutions have been proposed to the problem of providing through-the-earth communication. The simplest is to provide a wired link between the worker and the person with which they are communicating. However, this clearly restricts the worker to either the length of the wire or requires the worker be in a specific location, the terminating end of the wire, to initiate communication. It is also expensive to lay sufficient wire, particularly for very long tunnels and mines. Furthermore, in times of emergency, particularly a mine collapse or similar event, or even just due to wear and tear, wires are prone to sever, rendering the medium unusable.
Therefore, some wireless solutions have also been proposed for through-the-earth communication.
U.S. Pat. No. 7,043,204 to David Reagor et al. teaches a method and apparatus for effective through-the-earth communication involving a signal input device connected to a transmitter operating at a predetermined frequency sufficiently low to effectively penetrate useful distances through-the-earth, and having an analog to digital converter receiving the signal input and passing the signal input to a data compression circuit that is connected to an encoding processor, the encoding processor output being provided to a digital to analog converter. An amplifier receives the analog output from the digital to analog converter for amplifying said analog output and outputting said analog output to an antenna. A receiver having an antenna receives the analog output and passes the analog signal to a band pass filter whose output is connected to an analog to digital converter that provides a digital signal to a decoding processor whose output is connected to a data decompressor, the data decompressor providing a decompressed digital signal to a digital to analog converter. An audio output device receives the analog output from the digital to analog converter for producing audible output.
U.S. Pat. No. 7,149,472 to David Reagor et al. teaches a through-the-earth communication system that includes a digital signal input device; a transmitter operating at a predetermined frequency sufficiently low to effectively penetrate useful distances through-the earth; a data compression circuit that is connected to an encoding processor; an amplifier that receives encoded output from the encoding processor for amplifying the output and transmitting the data to an antenna; and a receiver with an antenna, a band pass filter, a decoding processor, and a data decompressor.
Both these patents provide a means of wireless through-the-earth communication. However, inherent in their designs is a requirement for relatively large and heavy equipment. In particular, the signal strength required to enable communications as taught by these patents requires a relatively large low frequency magnetic field to be generated by the transmitter. The magnetic field is generated by sending an alternating current around wire loop antenna, the size of the field being proportional to the magnitude of the current and the area of the wire loop. For a link of 500 ft, the current may be 10 A, and the loop area 50 square meters. The power supply for these devices is thus generally relatively large and heavy. Thus, at best a portable unit could be used as a receiver only. It is not generally possible for a worker to carry or easily move such a device if it were to have a transmitter. The size of the antenna further prevents adapting these patents for mobile use. It is also very difficult to transmit a voice signal at such low frequencies. For these reasons, neither of these two patents disclose a mobile implementation for a two way through-the-earth-radio.
U.S. Pat. No. 5,093,929 to Larry G. Stolarczyk et al. teaches a method for using an underground mine communication system to effect mine-wide communication and an intrinsically safe current limiter circuit for insuring that electrical equipment in the system will not cause incendiary conditions. The underground mine communication system comprises a plurality of repeaters and medium frequency radios, including mobile, portable and personal-carried radios, coupled to electrical conductors and natural waveguides existing in the earth, formed by coal seams, for example, by tuned loop antennas. Messages transmitted by the radios are carried to the repeaters by the conductors or natural waveguides. The repeaters amplify, replicate and retransmit the message at two different frequencies for transmission of the message to a surface base station and to other radios in the system. A paging system, which has a separate set of repeaters, is also coupled to the network of electrical conductors and natural waveguides by tuned loop antennas. The paging system alerts miners to contact the surface base station. Radios, pagers and repeaters in the system are equipped with the intrinsically safe current limiter circuit to preclude the development of incendiary conditions. The current limiter circuit comprises a series arrangement of a current trip circuit, a redundant current trip circuit and a current limiting field effect transistor controlled by a feedback control amplifier.
U.S. Pat. No. 4,777,652 to Larry G. Stolarczyk teaches a radio communications system operational within the medium frequency range (300 kHz to 3 MHz) for underground mine communications, coupling MF carrier signals into existing conductive channels extending to within the mine and devices within the mine and coupled to the conductive channel for receiving the MF carrier signals and communicating with portable and mobile transceivers operational within the MF frequency range and in the common seam mode. This patent requires a conductive channel, such as wire, for communicating a signal.
U.S. Pat. No. 7,050,831 to Zvi H. Meiksin et al. teaches a method and system for communication within an energy-transmission-limited environment. Multiple RF transceivers throughout the site are located site-wide such that areas within the site in which communications are desired are within range of at least one of the RF transceivers. At each location RF transceivers are connected to a control unit. The control unit provides power to the transceivers and allows bi-directional communication of audio/voice and/or digital information. The control units may be networked to each other using standard network type category-5 or equivalent cables and may communicate to one another via the network connection. The control units may also be networked via an alternating current power line by using an alternating current modem. The transceiver utilizes single sideband modulators to modulate voice and/or digital signals.
U.S. Pat. No. 4,710,708 to Louis H. Rorden et al. teaches a location method using relatively low frequency electromagnetic fields, e.g., 1-1000 Hz, for determining the relative position and/or orientation of a transmitting magnetic dipole antenna by using a vector field receiver. The transmitting antenna for subterranean location is preferably a single axis, elongated solenoid with a ferromagnetic core. The receiving sensor may be a precise three-axis magnetic field detector of either a magnetometer or search coil type. Measurements are made for one or more positions of either the transmitter or receiver, or with one or more transmitters or receivers. The relative location of the transmitter and the receiver is calculated with respect to some known survey station by a method of successive approximations. The operating frequency is chosen to minimize field distortion from common steel structures, such as pipe, casing or railroad tracks, and to minimize field scattering such as from conducting in homogeneities in the earth. Either the transmitter or receiver can be operated within metal structures such as casing. The method can be used for location of underground boreholes or pipelines; location of trapped miners; as a means of blind surveying such as in underground mines; or as a means of navigation such as in relatively shallow horizontal or vertical drilling and tunneling or in raise bore mining.
While each of these patents describes a mobile wireless implementation of through-the-earth communication, they all require some form of repeater or other fixed infrastructure to be used for relaying messages between the portable radio and the surface radio. The repeaters are in fixed location in the underground environment representing a large investment in infrastructure which may not be practical in underground environments that are only accessed occasionally, such as pipe-lines and sewers. In the event of an emergency, however, any or all of the repeaters could be damaged or even destroyed. Similarly, if the repeaters go out of service, the portable units are useless. In case of a cave or in a tunnel, the high frequency wireless communications between the repeaters and portable units would not work, as they would have to travel through rock. Additionally, similar to the fixed applications, these repeaters would also require large and heavy power supplies and are therefore not adaptable to portability.
U.S. Patent Publication No. 20080009242 to Mark Rhodes et al. teaches a data communication system that includes a transmitter, receiver and magnetically coupled antenna. The transmitter transmits data in a digitally modulated electromagnetic or magnetic signal. A receiver is provided for receiving the digitally modulated electromagnetic or magnetic signal. At least one of the transmitter and receiver is below ground and has a magnetically coupled antenna. Rhodes proposes a low frequency communications system very similar to those taught by Reagor. Rhodes proposes the use of simple loop antennas for transmitting, such that there is no new solution to the problems of power consumption and antenna size.
Furthermore, a person skilled in the art will understand that the antennae proposed by Rhodes ignore several fundamental issues related to the design of compact antennas for TTE communications. For example, Rhodes proposes the use of a simple, un-tuned loop antenna with a diameter of 1 m and 100 turns, without disclosing how such an antenna (which has an inductance of 52 mH and an impedance at the cited frequency of 5 kHz of +j1600 ohms) can be driven by a transmitter. To drive a current of only 1 A into this antenna, which would provide a Dipole Moment of only 100 Am2, would require a transmitter with an output voltage in excess of 1.5 kV. This is not practical for portable, or even fixed, equipment, as is used in an underground environment. Rhodes also teaches that antenna size may be reduced by using a coil in the form of a solenoid which encloses a higher permeability material such as ferrite. This does not address the problem of saturation of the core, which will occur at the levels of flux density needed to transmit over a useful distances, and is indeed the reason why this type of antenna has to date only been used as a receive antenna in TTE or other applications.
Other prior art is directed to low frequency antennas for underwater communications.
Great Britain Patent No. 2455909 to Mark Rhodes et al. teaches an antenna formed of multiple planar arrayed loops that is claimed to require a lower voltage power supply than an equivalent single loop on the basis that it has a lower inductance since currents in adjacent loops inside the antenna area cancel. However with this arrangement it is apparent that for a given Dipole Moment the power consumption of an antenna is increased in proportion to the number of arrays used to form the antenna. In a portable application, such as portable through-the-earth, it is essential to minimize the power consumption.
Great Britain Patent No. 2455653 to Mark Rhodes et al. teaches an antenna with multiple resonant loops, in which the loops are mutually coupled and tuned such that each resonates at a frequency close to the desired transmit frequency in order to increase the impedance bandwidth of the antenna. In this regard the antenna is very similar to a tuned band-pass filter in which the coupling between two resonators, closely spaced in frequency, is adjusted to obtain a given pass-band response. In a portable application, however, it is preferable to increase the Dipole Moment of the antenna and narrow the bandwidth of the primary antenna.
Great Britain Patent No. 2455654 to Mark Rhodes et al. teaches an electrically small antenna which is intended to synthesize a very low frequency plane wave by creating the E and H field components. However for a portable device used in above and below ground environments, the dielectric medium is air, and this requires the generation of a very high alternating voltage Which may create safety hazards in a unit that is worn on the body, as well as requiring a separate transmitter circuit.
Great Britain Patent No. 2455908 to Mark Rhodes et al. teaches a method whereby one or more ferrite receive antennas are placed in a direction orthogonal to the axis of a transmit loop antenna, in order to minimize the coupling between them, and thus to prevent the transmitter desensitizing the receiver. However it would be preferable to not require any particular orientation between the receive and transmit antennas.
Great Britain Patent No. 2455910 to Mark Rhodes et al. teaches a wearable antenna for low frequency underwater communications. It illustrates a number of antenna orientations in which the low frequency loop encircles a part of the user's body. The embodiments described have a number of practical disadvantages including restriction of the user's movements, stress imposed on the antenna windings by said movements, and the exposure of the user to the full strength of the magnetic field created by the loop antenna.
Thus, it is clear that there is a need to provide these workers with a means of reliably communicating through the earth and other physical barriers to wireless communication, while retaining their mobility, and without needing to rely on repeater equipment or other infrastructure. The primary challenge in providing such a technology is the minimization of the size and weight of the equipment, particularly for the transmitter and transmit antenna, to provide portability. Most importantly, both the power supply and the antenna structure for the transmitter must be small and light enough to be carried by a worker. To provide such features, a technique would have to be developed for enabling through-the-earth communication with significantly less transmission power than currently exists and a more efficient antenna design would be required. These have not been achieved to date.
Therefore, there is a need for a through-the-earth radio in which all aspects of the design are arranged such that the transmission power and antenna size are minimized to enable portability.